Design method for creep-fatigue strength of plate-fin heat exchanger

ABSTRACT

A design method for creep-fatigue strength of a plate-fin heat exchanger. The method includes preliminarily designing the plate-fin heat exchanger according to its service requirements, making a primary stress assessment for the plate-fin heat exchanger, calculating the equivalent mechanical and thermophysical parameters of the plate-fin heat exchanger core to satisfy the allowable stress requirement, performing a thermal fatigue analysis for the plate-fin heat exchanger based on these parameters and then calculating the fatigue life and creep life of the plate-fin heat exchanger to accomplish the comprehensive design of the plate-fin heat exchanger in the high-temperature service. The design method provides an effective method for the high temperature design of the plate-fin heat exchanger.

TECHNICAL FIELD

A design method for creep-fatigue strength of a plate-fin heat exchangerbelongs to the technical field of the heat exchangers.

BACKGROUND

With the development of technology, the energy consumption isincreasing, so energy saving and emission reduction as well as theimprovement of the energy utilization ratio have become focuses ofpublic attention. The heat-transfer equipment, as a core member of thehigh temperature system, not only requires efficient heat transferperformance, but also requires a compact structure. However, the mostcommonly used type of heat-transfer equipment at present is the shelland tube heat exchanger which has a large footprint and low heattransfer efficiency and it can be difficult to meet the requirements ofthe aerospace, high-temperature gas-cooled reactor, gas turbine andother fields using the shell and tube heat exchanger.

The plate-fin heat exchanger features a compact structure and high heattransfer efficiency. It is highly promising to study the plate-fin heatexchanger. However, the service environment of the plate-fin heatexchanger is getting worse and the high temperature and high pressureenvironment calls for increasingly strict design requirements for theplate-fin heat exchangers. The fracture mode is time-dependent for theservice at high temperature and alternating load and the current designcodes for pressure vessels are limited to the shell and tube heatexchangers and based on the elastic-plastic fracture mechanics (EPFM)theory, have neither considered the characteristics of the creep andfatigue fractures nor involved the brazing process, service environmentand other factors and cannot be directly adopted for the design of theplate-fin heat exchangers.

SUMMARY OF THE INVENTION

The technical problem to be solved by the disclosed design methodrelates to overcoming the shortcomings of the prior art by providing adesign method for creep-fatigue strength of a plate-fin heat exchanger,which makes it possible to conduct the high-temperature strength designfor the plate-fin heat exchanger in consideration of the brazingprocess, service environment, failure positions and other factors.

The technical solution adopted solves this technical problem by relatingto a design method for creep-fatigue strength of a plate-fin heatexchanger. The method includes the following steps:

-   -   Step 1: Preliminarily design the structure of the plate-fin heat        exchanger according to its design temperature and design        pressure requirements and define the operating temperature,        number of operating cycles and service life of the plate-fin        heat exchanger;    -   Step 2: Make a primary stress analysis for the plate-fin        structure with the finite element software to identify the        stress concentration parts and determine the allowable stress        S_(t);    -   Step 3: Judge whether the stress level of the stress        concentration parts satisfies the following conditions:

P _(m) ≤S _(t) ; P _(L) +P _(b) ≤K _(t) *S _(t);

-   -   -   Where, P_(m) means the primary membrane stress, P_(L) means            the local membrane stress, P_(b) means the primary bending            stress, S_(t) means the time-dependent allowable stress and            K_(t) assumes a value between 1.05 and 1.16;        -   If these conditions are satisfied, then perform Step 4; if            the primary stress is assessed unsatisfactory, change the            structure and plate thickness of the plate-fin heat            exchanger core and go back to Step 2;

    -   Step 4: Carry out the creep rupture experiment and fatigue        experiment on the plate-fin structure and on the aged base        material in the service environment, calculate the stress        magnification factor K_(σ) and the strain magnification factor        K_(s) and correct the fatigue design curve and creep rupture        design curve for the base material according to the experimental        results;

${K_{\sigma} = \frac{\sigma_{B}}{\sigma_{B}^{*}}},\mspace{11mu} {K_{s} = \frac{\Delta_{s}}{\Delta_{s}^{+}}},$

-   -   -   Where, σ_(B) and σ*_(B) mean the creep rupture strength of            the base material and plate-fin structure in the same creep            rupture time respectively,        -   Δ_(s) and Δ*_(t) mean the macro-strain range of the base            material and plate-fin structure in the same fatigue life            respectively;

    -   Step 5: Acquire the equivalent mechanical parameters and        equivalent thermophysical parameters of the plate-fin structure        thus to perform a finite element analysis for thermal fatigue        for the plate-fin heat exchanger, find the time history of the        micro-stress σ*_(th) of the plate-fin heat exchanger core in the        height direction and calculate the total strain Δε at the        fillet,

Δε=Δε_(ph) +K _(s)Δε*_(th),

-   -   -   Where, Δε_(ph) means the strain range that is derived from            the stress range Δσ_(ph) obtained from the primary stress            analysis;        -   Δε*_(th) means the ratio of the difference between the            maximum value and the minimum value of the macroscopic            stress σ*_(th) obtained from the thermal fatigue analysis to            the elastic modulus of the plate-fin heat exchanger core in            the height direction;

    -   Step 6: Calculate the fatigue damage D_(f) and creep damage        D_(c) of the plate-fin heat exchanger core,

${D_{f} = \frac{N_{t}}{N_{f}\left( {\Delta \; ɛ*K_{s}} \right)}},$

-   -   -   Where, N_(t) means the number of fatigue cycles,        -   N_(f)(ε) means the corresponding fatigue life on the            corrected fatigue design curve if the strain range is ε;

${D_{c} = {N_{i}*{\int_{0}^{t_{R}}\frac{dt}{{tr}\left\lbrack {{\sigma_{e}^{*}(t)}*K_{\sigma}} \right\rbrack}}}},$

-   -   -   Where, N_(i) means the number of fatigue cycles,        -   t_(h) means the strain retention time,        -   σ*_(ε)(t) means the macro stress at the moment, t,        -   tr(σ) means the corresponding creep rupture life on the            corrected creep rupture design curve if the stress is σ;

    -   Step 7: If D_(f)+D_(c) is less than 1, then perform Step 8; if        D_(f)+D_(c) is greater than or equal to 1, then perform Step 1;        and

    -   Step 8: The design for the plate-fin heat exchanger is        completed.

-   The allowable stress S_(t) as described in Step 2 and Step 3    includes the allowable stress S_(t1) of the fin area and the    allowable stress S_(t2) of the seal area.

The step of acquiring the equivalent mechanical parameters andequivalent thermophysical parameters of the plate-fin structure asdescribed in Step 5 comprises the substeps of:

-   -   a. Dividing the plate-fin heat exchanger core into several        plate-fin cells of the same shape;    -   b. Considering the plate-fin cells equivalent to uniform solid        plates;    -   c. Acquiring the equivalent mechanical parameters and equivalent        thermophysical parameters of a plate-fin cell, thus obtaining        the equivalent mechanical parameters and equivalent        thermophysical parameters of the whole plate-fin heat exchanger        core.

The said equivalent mechanical parameters include the anisotropicequivalent elastic modulus, equivalent shear modulus and Poisson'sratio; the said equivalent thermophysical parameters include theequivalent thermal conductivity, equivalent coefficient of thermalexpansion, equivalent density and equivalent specific heat.

The said anisotropic equivalent elastic modulus is calculated asfollows:

-   -   A coordinate system is established by taking the midpoint of the        bottom of the front of the plate-fin heat exchanger as its        origin, taking the direction parallel to the axis of the flow        path (3) in a horizontal plane as its x-axis, taking the        direction perpendicular to the axis of the flow path (3) as its        y-axis and taking the vertical direction as its z-axis,

${E_{x} = {\frac{\left\{ {\left\lbrack {{l\; \tan \; \frac{a}{2}} + {dt} + {\left( {l + d} \right)\delta}} \right\rbrack - {{\delta \left( {\delta + t} \right)}{\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}}} \right\}}{\left\lbrack {d - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} + {l\; \sin \; \frac{a}{2}}} \right\rbrack \left( {{l\; \cos \; \frac{a}{2}} + \delta + {2t}} \right)}E_{0}}},{E_{y} = {\frac{{2t} + \delta}{{l\; \cos \; \frac{a}{2}} + \delta + {2t}}E_{0}}},{E_{z} = {\frac{{\delta \left\lbrack {l + {\delta \; \tan \; \frac{a}{2}} + {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}}} \right\rbrack}\cos \; \frac{a}{2}}{\left\lbrack {d - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} + {l\; \sin \; \frac{a}{2}}} \right\rbrack \left( {{l\; \cos \; \frac{a}{2}} + \delta} \right)}E_{0}}},$

-   -   Where, E_(x), E_(y) and E_(z) mean the equivalent elastic        modulus in the direction of the x-axis, y-axis and z-axis        respectively,    -   E₀ means the elastic modulus of the base material,    -   d means the width of the upper side of the flow path (3) of the        plate-fin heat exchanger core,

$\frac{a}{2}$

-   -   means the angle between the side and vertical plane of the flow        path (3),    -   l means the length of the side of the flow path (3),    -   t means the thickness of the plate (1) of the plate-fin heat        exchanger core,    -   δ means the thickness of the fin (2) of the plate-fin type heat        exchanger core.

The Poisson's ratio is calculated as follows:

${v_{xz} = {\frac{E_{z}}{E_{x}}v_{0}}},{v_{xy} = {\frac{E_{y}}{E_{x}}v_{0}}},{v_{yz} = \frac{\left\{ {{{v_{0}\left( {{2t} + \delta} \right)}\cos \; \frac{a}{2}} + {\left( {{l\; \cos \; \frac{a}{2}} + \delta} \right)\sin \; \frac{a}{2}\tan \; \frac{a}{2}}} \right\} E_{z}}{\cos \; \frac{a}{2}\left( {{2t} + \delta + {l\; \cos \; \frac{a}{2}}} \right)E_{y}}},$

-   -   Where, v_(xy) means the ratio of the x-axis strain to the y-axis        strain under the y-axis load,    -   v_(xz) means the ratio of the x-axis strain to the z-axis strain        under the z-axis load,    -   v_(yz) means the ratio of the y-axis strain to the z-axis strain        under the z-axis load,    -   v₀ means the Poisson's ratio of the base material.

The said equivalent shear modulus is calculated as follows:

-   -   A coordinate system is established by taking the midpoint of the        bottom of the front of the plate-fin heat exchanger as its        origin, taking the direction parallel to the axis of the flow        path (3) in a horizontal plane as its x-axis, taking the        direction perpendicular to the axis of the flow path (3) as its        y-axis and taking the vertical direction as its z-axis,

$\mspace{79mu} {{G_{xy} = \frac{\left( {{2t} + \delta} \right)E_{0}}{2\left( {{l\; \cos \; \frac{a}{2}} + \delta + {2t}} \right)\left( {1 + v_{0}} \right)}},\mspace{79mu} {G_{xz} = \frac{\left( {{\left\lbrack {d - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} + {l\; \sin \; \frac{a}{2}}} \right\rbrack \left\lbrack {{\delta \; \tan \; \frac{a}{2}} + {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}}} \right\rbrack}E_{0}} \right)}{\left( {1 + v_{0}} \right)\begin{Bmatrix}{{\left( {{2t} + \delta} \right)\left\lbrack {{\delta \; \tan \; \frac{a}{2}} + {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}}} \right\rbrack} +} \\{l\left\lbrack {d - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} + {l\; \sin \; \frac{a}{2}}} \right\rbrack}\end{Bmatrix}}},{G_{yz} = \frac{2E_{0}{\delta^{3}\left( {{l\; \cos \; \frac{a}{2}} + \delta + {2t}} \right)}}{{{4\; {\delta^{3}\left( {1 + v_{0}} \right)}\left( {{2t} + \delta} \right)} + {{\left( {l - {2\; \delta}} \right)^{3}\left\lbrack {d - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} + {l\; \sin \; \frac{a}{2}}} \right\rbrack}\cos \; \frac{a}{2}}}\;}},}$

-   -   Where, v₀ and E₀ mean the Poisson's ratio and elastic modulus of        the base material, respectively,    -   d means the width of the upper side of the flow path (3) of the        plate-fin heat exchanger core,

$\frac{a}{2}$

-   -   means the angle between the side and the vertical plane of the        flow path (3),    -   l means the length of the side of the flow path (3),    -   t means the thickness of the plate (1) of the plate-fin heat        exchanger core,    -   δ means the thickness of the fin (2) of the plate-fin type heat        exchanger core.

The said equivalent thermal conductivity is calculated as follows:

-   -   A coordinate system is established by taking the midpoint of the        bottom of the front of the plate-fin heat exchanger as its        origin, taking the direction parallel to the axis of the flow        path (3) in a horizontal plane as its x-axis, taking the        direction perpendicular to the axis of the flow path (3) as its        y-axis and taking the vertical direction as its z-axis,

${\lambda_{x} = {\frac{1}{\left\lbrack {{l\; \sin \; \frac{a}{2}} + d - {\delta \; {\sin \left( {{45{^\circ}} - \frac{a}{4}} \right)}}} \right\rbrack \left( {{l\; \cos \; \frac{a}{2}} + \delta + {2t}} \right)}\begin{Bmatrix}{{\lambda_{a}l\; \cos \; {\frac{a}{2}\left\lbrack {d - {2\; \delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} + {l\; \sin \; \frac{a}{2}}} \right\rbrack}} +} \\{\lambda_{m}{\delta \left\lbrack {l - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} + d} \right\rbrack}}\end{Bmatrix}}},{\frac{1}{\lambda_{y}} = {\frac{1}{{l\; \sin \; \frac{a}{2}} + d - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}}}\left\{ {\frac{\delta}{\lambda_{m}} + \frac{\left\lbrack {{l\; \sin \; \frac{a}{2}} + d - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} - \delta} \right\rbrack \left( {{l\; \cos \; \frac{a}{2}} + \delta + {2t}} \right)}{{\lambda_{a}l\; \cos \; \frac{a}{2}} + {\lambda_{m}\left( {{2t} + \delta} \right)}}} \right\}}},{\frac{1}{\lambda_{z}} = {\frac{1}{{l\; \cos \; \frac{a}{2}} + \delta + {2t}}\left\{ {\frac{\delta + {2t}}{\lambda_{m}} + \frac{l\; \cos \; {\frac{a}{2}\left\lbrack {d - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} + {l\; \sin \; \frac{a}{2}}} \right\rbrack}}{{\lambda_{a}\left\lbrack {d - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} + {l\; \sin \; \frac{a}{2}} - \delta} \right\rbrack} + {\lambda_{m}\delta}}} \right\}}},$

-   -   Where, λ_(x), λ_(y) and λ_(z) mean the equivalent thermal        conductivity in the direction of the x-axis, y-axis and z-axis,    -   λ_(a) and λ_(m) mean the thermal conductivity of the base        material and air respectively,    -   d means the width of the upper side of the flow path (3) of the        plate-fin heat exchanger core,

$\frac{a}{2}$

-   -   means the angle between the side of the flow path (3) and the        vertical plane of the flow path (3),    -   l means the length of the side of the flow path (3),    -   t means the thickness of the plate (1) of the plate-fin heat        exchanger core,    -   δ means the thickness of the fin (2) of the plate-fin type heat        exchanger core;

The said equivalent coefficient of thermal expansion is calculated asfollows:

A coordinate system is established by taking the midpoint of the bottomof the front of the plate-fin heat exchanger as its origin, taking thedirection parallel to the axis of the flow path (3) in a horizontalplane as its x-axis, taking the direction perpendicular to the axis ofthe flow path (3) as its y-axis and taking the vertical direction as itsz-axis,

${\alpha_{z} = {\frac{\alpha_{0}}{{l\; \cos \; \frac{a}{2}} + {2t} + \delta}\left\{ {{2t} + \frac{\begin{matrix}{{\left( {{l\; \sin \; \frac{a}{2}} + {\delta \; \tan \; \frac{a}{2}}} \right)\delta^{2}} + \left\lbrack {d - {2\; \delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}}} \right\rbrack} \\{{\left\lbrack {d - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} + {l\; \sin \; \frac{a}{2}}} \right\rbrack \delta}\;}\end{matrix}}{{\left\lbrack {d - {2\; \delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}}} \right\rbrack \left\lbrack {d - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} + {l\; \sin \; \frac{a}{2}}} \right\rbrack} + {\delta^{2}\tan \; \frac{a}{2}}}} \right\}}},\mspace{20mu} {\alpha_{y} = \alpha_{0}},\mspace{20mu} {\alpha_{x} = \alpha_{0}},$

-   -   Where, a₀ means the equivalent coefficient of thermal expansion        of the base material,    -   d means the width of the upper side of the flow path (3) of the        plate-fin heat exchanger core,

$\frac{a}{2}$

-   -   means the angle between the side of the flow path (3) and the        vertical plane,    -   l means the length of the side of the flow path (3),    -   t means the thickness of the plate (1) of the plate-fin heat        exchanger core,    -   δ means the thickness of the fin (2) of the plate-fin type heat        exchanger core.

The equivalent density and equivalent specific heat are calculated asfollows:

${\beta_{a} = \frac{{l^{2}\sin \; \frac{a}{2}\cos \; \frac{a}{2}} + {l\; \cos \; {\frac{a}{2}\left\lbrack {d - {2\; \delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}}} \right\rbrack}}}{\left( {{l\; \cos \; \frac{a}{2}} + \delta + {2t}} \right)\left\lbrack {d + {l\; \sin \; \frac{a}{2}} - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}}} \right\rbrack}},{\beta_{m} = {1 - \beta_{a}}},{\overset{\_}{\rho} = {{\beta_{m}\rho_{m}} + {\beta_{a}\rho_{a}}}},{C_{p} = \frac{{\left( {1 - \beta_{a}} \right)\rho_{m}c_{p\; 1}} + {\beta_{a}\rho_{a}c_{p\; 2}}}{{\left( {1 - \beta_{a}} \right)\rho_{m}} + {\beta_{a}\rho_{a}}}},$

-   -   Where, β_(m) and β_(a) mean the base material volume fraction        and air volume fraction respectively,    -   ρ_(m) and ρ_(a) mean the base material density and air density        respectively,    -   c_(p1) and c_(p2) mean the base material specific heat and air        specific heat respectively,    -   c_(p) and ρ mean the equivalent specific heat and equivalent        density respectively,    -   d means the width of the upper side of the flow path (3) of the        plate-fin heat exchanger core,

$\frac{a}{2}$

-   -   means the angle between the side and the vertical plane of the        flow path (3),    -   l means the length of the side of the flow path (3),    -   t means the thickness of the plate (1) of the plate-fin heat        exchanger core,    -   δ means the thickness of the fin (2) of the plate-fin type heat        exchanger core.

Compared with the prior arts, the present invention has the followingbeneficial effects:

-   1. The design method for creep-fatigue strength of a plate-fin heat    exchanger comprehensively considers the influences of the brazing    process, service environment, failure positions and other factors of    the plate-fin heat exchanger, conducts equivalent homogenization for    the plate-fin heat exchanger core, calculates the equivalent    mechanical parameters and equivalent thermophysical parameters of    the plate-fin heat exchanger, solves the problem that the plate-fin    heat exchanger cannot be designed for high temperature strength    directly with the finite element software owing to its complex    periodic structure, provides a theoretical basis for the    high-temperature strength design for the plate-fin heat exchanger,    thus making an effective life prediction for the plate-fin heat    exchangers servicing at high temperature and alternating load and    providing an effective method for the design of the plate-fin heat    exchanger servicing at high temperature.-   2. Equivalent homogenization divides the plate-fin structure into    the same plate-fin cells and considers the plate-fin cells    equivalent to uniform solid plates so as to calculate the equivalent    mechanical parameters and equivalent thermophysical parameters of    the plate-fin structure, thereby solving the problem that it is    difficult to perform the finite element simulation for the plate-fin    heat exchanger owing to the periodic complex structure of the    plate-fin heat exchanger so that the thermal fatigue analysis for    the plate-fin heat exchanger is performed subsequently with the    finite element analysis software.-   3. The equivalent mechanical parameters and equivalent    thermophysical parameters of the plate-fin heat exchanger core are    given as an analytic expression, solving the problem that the    parameters have to be acquired only through the complex computer    simulation or experiment before, facilitating the calculation of the    effective parameters and greatly improving the design efficiency of    the plate-fin heat exchangers.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a main view for the plate-fin heat exchanger core.

FIG. 2 illustrates a main view for the plate-fin cells.

FIG. 3 illustrates a broken-line graph for the creep-fatigue lifeevaluation.

SPECIFIC EMBODIMENTS

The preferred embodiments of this disclosure are illustrated in FIGS. 1to 3.

The design method for creep-fatigue strength of a plate-fin heatexchanger includes the following steps:

-   Step 1: Preliminarily design the structure of the plate-fin heat    exchanger according to its design temperature and design pressure    requirements and define the operating temperature, number of    operating cycles and service life of the plate-fin heat exchanger.    -   The number of operating cycles is the product of the design        service life and the number of annual shutdown; the service life        is the design life.        -   As shown in FIGS. 1 to 2, the plate-fin heat exchanger core            includes plates 1 and fins 2, with the fins 2 provided            between every two adjacent plates 1, and is formed through            superposing and brazing several plates 1 and fins 2 in a            staggered fashion, thus forming several flow paths 3 between            every two adjacent plates 1 and the cross section of the            flow paths 3 is an isosceles trapezoid.-   Step 2: Make a primary stress analysis for the plate-fin structure    with the finite element software to identify the stress    concentration parts and determine the allowable stress S_(t).    -   The influence of the brazing seam on the structural stress is        not taken into account during the analysis, and the brazing        bonding rate is assumed to be 100%. Then, the method involves        considering the thermal aging of the material in the process of        brazing, service environment (such as the influence of the        helium atmosphere on the material strength in the high        temperature gas-cooled reactor) and seal structure, and carrying        out the experimental analysis to determine the allowable stress.    -   The allowable stress S_(t) is the time-dependent allowable        stress and the allowable stress S_(t) includes the allowable        stress S_(t1) in the fin area and the allowable stress S_(t2) in        the seal area. For acquiring the allowable stress S_(t1) in the        fin area, uniaxial tensile and creep rupture experiments are        carried out on the aged base material in the service environment        (such as helium atmosphere) for the brazing high temperature and        service environment. For acquiring the allowable stress S_(t2)        in the seal area, it is necessary to carry out the tensile        strength experiment on the filler metal. Finally, according to        the ASME design criteria and correction results, the method        involves determining the allowable stress S_(t1) in the fin area        and the allowable stress S_(t2) in the seal area.    -   The allowable stress of the fin area takes the minimum value of        the following four factors:        -   {circle around (1)} Yield stress at design            temperature*(1/1.1)*p,        -   {circle around (2)} 67% of minimal stress resulting in the            creep rupture *q,        -   {circle around (3)} 80% of minimal stress resulting in the            start of the creep at the third stage *q, and        -   {circle around (4)} Minimal stress up to 1% of the total            strain (elasticity, plasticity and creep).    -   The strain in the seal structure is limited, the allowable        strain takes one third of the fracture elongation of the filler        metal and the allowable stress of the seal area takes the        minimum of the following three factors:        -   {circle around (2)} Base material yield stress at the design            temperature*(1/1.1)*p,        -   {circle around (3)} Allowable strain * base material elastic            modulus, and        -   {circle around (4)} 67% of minimal stress generated by creep            rupture *q.    -   Where, p means the ratio of the yield stress of the aged base        material to that of the un-aged base material; and    -   q means the ratio of the creep rupture strength of the aged base        material to that of the unaged base material.-   Step 3: Judge whether the stress level of the stress concentration    parts satisfies the following conditions:

P _(m) ≤S _(t) ; P _(L) +P _(b) ≤K _(t) *S _(t).

-   -   Where, P_(m) means the primary membrane stress, P_(L) means the        local membrane stress, P_(b) means the primary bending stress,        S_(t) means the time-dependent allowable stress and K_(t) takes        a value between 1.05 and 1.16.    -   If these conditions are satisfied, perform Step 4. If the        primary stress is assessed unsatisfactory, change the structure        and plate thickness of the plate-fin heat exchanger core and go        back to Step 2.    -   The conditions for satisfying the above-mentioned judgment        conditions are as follows: the plate-fin structure fails when        the stress of the stress concentration parts in the fin area        reaches the allowable stress of the fin area; the seal structure        fails when stress of the stress concentration parts in the seal        structure reaches the allowable stress of the seal area. Failure        of any of the plate-fin structure and seal structure will result        in the failure of the plate-fin heat exchanger core, which needs        to be redesigned to reduce the structural stress level.    -   The stress is assessed with the allowable stress S_(t1) of the        fin area and the allowable stress S_(t2) of the seal area,        respectively. If the stress of the fin area satisfies the        allowable stress S_(t1) and the allowable stress of the seal        area satisfies the allowable stress S_(t2), the stress is        assessed satisfactory. If the stress of the fin area does not        satisfy the allowable stress S_(t1) or the stress of the seal        area does not satisfy the allowable stress S_(t2), the stress is        assessed unsatisfactory.

-   Step 4: Carry out the creep rupture experiment and fatigue    experiment on the plate-fin structure and on the aged base material    in the service environment, calculate the stress magnification    factor K_(σ) and the strain magnification factor K_(s) and correct    the fatigue design curve and creep rupture design curve for the base    material according to the experimental results.

${K_{\sigma} = \frac{\sigma_{B}}{\sigma_{B}^{*}}},\mspace{14mu} {K_{s} = {\frac{\Delta_{s}}{\Delta_{s}^{+}}.}}$

-   -   Where, σ_(B) and σ*_(B) mean the creep rupture strength of the        base material and plate-fin structure in the same creep rupture        time, respectively,        -   Δ_(s) and Δ*_(s) mean the macro-strain range of the base            material and plate-fin structure in the same fatigue life,            respectively.

-   Step 5: Acquire the equivalent mechanical parameters and equivalent    thermophysical parameters of the plate-fin structure so as to make a    finite element analysis for thermal fatigue for the plate-fin heat    exchanger.    -   The plates 1 and fins 2 of the plate-fin heat exchanger core are        formed through brazing, compact in structure and different from        the conventional homogeneous material. The plate-fin type heat        exchanger core features a periodic porous structure and the        periodicity and complexity of the core structure of the heat        exchanger makes it difficult to directly make the finite element        analysis which must be made with the equivalent homogenization        method. In order to make the finite element analysis for the        plate-fin heat exchanger core, the homogenization method has        been introduced.    -   The homogenization method means that the composite has a regular        or approximate regular structure, this fairly regular        heterogeneous material can be assumed to have a periodic        structure, and it should be emphasized that these        non-homogeneous materials are very small compared to the size of        the composite. In view of this, such types of materials are        sometimes referred to as the composites with periodic        microstructures. However, it is quite difficult to analyze these        boundary values containing a large number of heterogeneous        materials even using modern high-speed computers. To overcome        this difficulty, it is necessary to find a method to replace the        composite with an equivalent material model and this process is        called homogenization. The essence of homogenization is to        replace the composite of periodic structure with the equivalent        material and acquiring the performance parameters of the        equivalent material is the key step of homogenization.    -   The step of acquiring the equivalent mechanical parameters and        equivalent thermophysical parameters of the plate-fin heat        exchanger core include the following substeps:        -   Step a. Divide the plate-fin heat exchanger core into            several plate-fin cells of the same shape.            -   In the present embodiment, the structure shown in FIG. 2                is a plate-fin cell so that the plate-fin type heat                exchanger core can be regarded as a combination of a                plurality of plate-fin cells.        -   Step b. Consider the plate-fin cells equivalent to uniform            solid plates.            -   Since the structure of the plate-fin cells is not                uniform, the plate-fin cells are regarded as a                homogeneous material, that is, the plate-fin cells are                considered equivalent to uniform solid plates so as to                replace the nonuniform plate-fin structure with                equivalent solid plates.        -   Step c. Acquire the equivalent mechanical parameters and            equivalent thermophysical parameters of a plate-fin cell so            as to obtain the equivalent mechanical parameters and            equivalent thermophysical parameters of the whole plate-fin            heat exchanger core. The equivalent mechanical parameters            and equivalent thermophysical parameters here may also be            acquired through the finite element analysis software or            experiments.            -   The equivalent mechanical parameters include the                anisotropic equivalent elastic modulus, equivalent shear                modulus and Poisson's ratio. The equivalent                thermophysical parameters include the equivalent thermal                conductivity, equivalent coefficient of thermal                expansion, equivalent density and equivalent specific                heat. A coordinate system is established by taking the                midpoint of the bottom of the front of the plate-fin                heat exchanger as its origin, taking the direction                parallel to the axis of the flow path 3 in a horizontal                plane as its x-axis, taking the direction perpendicular                to the axis of the flow path 3 as its y-axis and taking                the vertical direction as its z-axis so as to calculate                the equivalent mechanical parameters and equivalent                thermophysical parameters of the plate-fin heat                exchanger core.            -   The equivalent elastic modulus of the plate-fin heat                exchanger core is calculated as follows:            -   The equivalent elastic modulus in the direction of the                z-axis is calculated with the balance between the force                applied on the plates 1 and the force applied on the                vertical portion of the fins 2,

$E_{s} = {\frac{{\delta \left\lbrack {l + {\delta \; \tan \frac{a}{2}} + {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}}} \right\rbrack}\cos \frac{a}{2}}{\left\lbrack {d - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} + {l\; \sin \frac{a}{2}}} \right\rbrack \left( {{l\; \cos \frac{a}{2}} + \delta} \right)}{E_{0}.}}$

-   -   -   -   The equivalent elastic modulus in the direction of the                x-axis is calculated with the concepts of the equivalent                stress and actual strain,

$E_{x} = {\frac{\left\{ {\left\lbrack {{l\; \tan \frac{a}{2}} + {dt} + {\left( {l + d} \right)\delta}} \right\rbrack - {{\delta \left( {\delta + t} \right)}{\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}}} \right\}}{\left\lbrack {d - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} + {l\; \sin \frac{a}{2}}} \right\rbrack \left( {{l\; \cos \frac{a}{2}} + \delta + {2\; t}} \right)}{E_{0}.}}$

-   -   -   -   The equivalent elastic modulus in the direction of the                y-axis is calculated with the concepts of the equivalent                stress and actual strain,

$E_{y} = {\frac{{2t} + \delta}{{l\; \cos \frac{a}{2}} + \delta + {2t}}{E_{0}.}}$

-   -   -   -   Where, E_(x), E_(y) and E_(z) mean the equivalent                elastic modulus in the direction of the x-axis, y-axis                and z-axis respectively.            -   E₀ means the elastic modulus of the base material.            -   d means the width of the upper side of the flow path 3                of the plate-fin heat exchanger core.

$\frac{a}{2}$

-   -   -   -   means the angle between the side and vertical plane of                the flow path 3.            -   l means the length of the side of the flow path 3.            -   t means the thickness of the plate 1 of the plate-fin                heat exchanger core.            -   δ means the thickness of the fin 2 of the plate-fin type                heat exchanger core.            -   The Poisson's ratio of the plate-fin heat exchanger core                is calculated as follows:            -   In consideration of the reinforcing function of the                plates, to calculate v_(xy), firstly calculate v_(yx),                namely, calculate the ratio of the y-axis strain to the                x-axis strain under the x-axis load and then derive                v_(xy) from the relationship between the elastic modulus                and Poisson's ratio,            -   Namely, from

${\frac{v_{xy}}{E_{y}} = \frac{v_{yx}}{E_{x}}},$

-   -   -   -   Derive

${v_{xy} = {\frac{E_{y}}{E_{x}}v_{0}}},$

-   -   -   -   Similarly, we have:

${v_{xz} = {\frac{E_{z}}{E_{x}}v_{0}}},{v_{yz} = \frac{\left\{ {{{v_{0}\left( {{2t} + \delta} \right)}\cos \frac{a}{2}} + {\left( {{l\; \cos \frac{a}{2}} + \delta} \right)\sin \frac{a}{2}\tan \frac{a}{2}}} \right\} E_{z}}{\cos \frac{a}{2}\left( {{2t} + \delta + {l\; \cos \frac{a}{2}}} \right)E_{y}}},$

-   -   -   -   Where, v_(xy) means the ratio of the x-axis strain to                the y-axis strain under the y-axis load,            -   v_(xz) means the ratio of the x-axis strain to the                z-axis strain under the z-axis load,            -   v_(yz) means the ratio of the y-axis strain to the                z-axis strain under the z-axis load, and            -   v₀ means the Poisson's ratio of the base material.            -   The equivalent shear modulus of the plate-fin heat                exchanger core is calculated as follows:            -   To calculate

$G_{xy},{G = \frac{E}{2\left( {1 + v} \right)}}$

-   -   -   -   is known for each isotropic homogeneous material,            -   Thus find:

${G_{xy} = \frac{\left( {{2t} + \delta} \right)E_{0}}{2\left( {{l\; \cos \frac{a}{2}} + \delta + {2t}} \right)\left( {1 + v_{0}} \right)}},$

-   -   -   -   Calculate G_(xz) and G_(yz), where both G_(xz) and                G_(yz) mean the ratio of the equivalent shear stress to                the actual shear strain,            -   Thus find:

$\mspace{79mu} {{G_{xz} = \frac{{\left\lbrack {d - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} + {l\; \sin \frac{a}{2}}} \right\rbrack \left\lbrack {{\delta \; \tan \frac{a}{2}} + {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}}} \right\rbrack}E_{0}}{\left( {1 + v_{0}} \right)\begin{Bmatrix}{{\left( {{2t} + \delta} \right)\left\lbrack {{\delta \; \tan \frac{a}{2}} + {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}}} \right\rbrack} +} \\{l\left\lbrack {d - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} + {l\; \sin \frac{a}{2}}} \right)}\end{Bmatrix}}},{G_{yz} = \frac{2E_{0}{\delta^{3}\left( {{l\; \cos \frac{a}{2}} + \delta + {2t}} \right)}}{{4\; {\delta^{3}\left( {1 + v_{0}} \right)}\left( {{2t} + \delta} \right)} + {{\left( {l - {2\; \delta}} \right)^{3}\left\lbrack {d - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} + {l\; \sin \frac{a}{2}}} \right\rbrack}\cos \frac{a}{2}}}},}$

-   -   -   -   Where, v₀ and E₀ mean the Poisson's ratio and elastic                modulus of the base material, respectively.            -   The equivalent thermal conductivity of the plate-fin                heat exchanger core is calculated as follows:            -   On the basis of the law of minimal thermal resistance,                also known as the parallel law, when the heat is                transferred in the object, the heat flow is passed along                the channel with the least resistance, or the channel                has a minimum thermal resistance state when the heat                flow passes through the directional heat flow, and the                total heat resistance of the corresponding channel is                minimal thermal resistance, also known as equivalent                thermal resistance. On the basis of the law of the                equivalent thermal conductivity, when only the heat                transfer is considered, and specific equivalent thermal                resistance of single element of the composite is                considered equal to the total thermal resistance of the                composite, then the equivalent thermal conductivity of                that single element is considered equal to the total                thermal conductivity of the composite regardless of the                size of the element.            -   From the description above, we can see that, to find the                thermal conductivity of the whole plate-fin heat                exchanger core, it is only necessary to find the                equivalent thermal conductivity of a plate-fin cell.

${\lambda_{x} = {\frac{1}{\left\lbrack {{l\; \sin \frac{a}{2}} + d - {\delta \; {\sin \left( {{45{^\circ}} - \frac{a}{4}} \right)}}} \right\rbrack \left( {{l\; \cos \frac{a}{2}} + \delta + {2t}} \right)}\begin{Bmatrix}{{\lambda_{a}l\; \cos {\frac{a}{2}\left\lbrack {d - {2\; \delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} + {l\; \sin \frac{a}{2}}} \right\rbrack}} +} \\{\lambda_{m}{\delta \left\lbrack {l - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} + d} \right\rbrack}}\end{Bmatrix}}},{\frac{1}{\lambda_{y}} = {\frac{1}{{l\; \sin \frac{a}{2}} + d - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}}}\left\{ {\frac{\delta}{\lambda_{m}} + \frac{\left\lbrack {{l\; \sin \frac{a}{2}} + d - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} - \delta} \right\rbrack \left( {{l\; \cos \frac{a}{2}} + \delta + {2t}} \right)}{{\lambda_{a}l\; \cos \frac{a}{2}} + {\lambda_{m}\left( {{2t} + \delta} \right)}}} \right\}}},{\frac{1}{\lambda_{z}} = {\frac{1}{{l\; \cos \frac{a}{2}} + \delta + {2t}}\left\{ {\frac{\delta + {2t}}{\lambda_{m}} + \frac{l\; \cos {\frac{a}{2}\left\lbrack {d - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} + {l\; \sin \frac{a}{2}}} \right\rbrack}}{{\lambda_{a}\left\lbrack {d - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} + {l\; \sin \frac{a}{2}} - \delta} \right\rbrack} + {\lambda_{m}\delta}}} \right\}}},$

-   -   -   -   Where λ_(x), λ_(y) and λ_(z) mean the equivalent thermal                conductivity in the direction of the x-axis, y-axis and                z-axis, respectively.            -   λ_(a) and λ_(m) mean the thermal conductivity of the                base material and air, respectively.            -   The equivalent coefficient of thermal expansion of the                plate-fin heat exchanger core is calculated as follows:            -   The top and bottom plates 1 of a plate-fin cell can                expand freely. The fins 2 and the horizontal part and                inclined part of the fins 2 interact with each other due                to the difference in the expansion in the direction of                the z-axis, so we have:

$\alpha_{z} = {\frac{\alpha_{0}}{{l\; \cos \frac{a}{2}} + {2t} + \delta}{\left\{ {{2t} + \frac{\begin{matrix}{{\left( {{l\; \sin \frac{a}{2}} + {{\delta tan}\frac{a}{2}}} \right)\delta^{2}} + \left\lbrack {d - {2\; \delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}}} \right\rbrack} \\{\left\lbrack {d - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} + {l\; \sin \frac{a}{2}}} \right\rbrack \delta}\end{matrix}}{\begin{matrix}\left\lbrack {d - {2\; \delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}}} \right\rbrack \\{\left\lbrack {d - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}} + {l\; \sin \frac{a}{2}}} \right\rbrack + {\delta^{2}\tan \frac{a}{2}}}\end{matrix}}} \right\}.}}$

-   -   -   -   The plates 1 and the fins 2 have the same thermal                expansion in the direction of the y-axis and x-axis, so                we have:

a_(y)=a₀,

a_(x)=a₀,

-   -   -   -   Where, a₀ means the equivalent coefficient of thermal                expansion of the base material.            -   The equivalent density and equivalent specific heat of                the plate-fin heat exchanger core are calculated as                follows:

${\beta_{a} = \frac{{l^{2}\sin \frac{a}{2}\cos \frac{a}{2}} + {l\; \cos {\frac{a}{2}\left\lbrack {d - {2\; \delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}}} \right\rbrack}}}{\left( {{l\; \cos \frac{a}{2}} + \delta + {2t}} \right)\left\lbrack {d + {l\; \sin \frac{a}{2}} - {\delta \; {\tan \left( {{45{^\circ}} - \frac{a}{4}} \right)}}} \right\rbrack}},{\beta_{m} = {1 - \beta_{a}}},{\overset{\_}{\rho} = {{\beta_{m}\rho_{m}} + {\beta_{a}\rho_{a}}}},{c_{p} = \frac{{\left( {1 - \beta_{a}} \right)\rho_{m}c_{p\; 1}} + {\beta_{a}\rho_{a}c_{p\; 2}}}{{\left( {1 - \beta_{a}} \right)\rho_{m}} + {\beta_{a}\rho_{a}}}},$

-   -   -   -   Where, β_(m) and β_(a) mean the base material volume                fraction and air volume fraction, respectively.            -   ρ_(m) and ρ_(a) mean the base material density and air                density, respectively.            -   c_(p1) and c_(p2) mean the base material specific heat                and air specific heat, respectively.            -   c_(p) and ρ mean the equivalent specific heat and                equivalent density, respectively.            -   Perform the anisotropic elastic analysis for thermal                fatigue with the homogenization method through the                finite element analysis software using the calculated                equivalent mechanical parameters and equivalent                thermophysical parameters. Derive the superposition                direction of the plate-fin structure from the results of                thermal stress analysis, namely, the time history of the                macroscopic stress σ*_(th) in the direction of the                z-axis, Δε*_(th) is the difference between the maximum                value and the minimum value of the macroscopic stress,                and the ratio of the difference to the elastic modulus                in the direction of the z-axis is the strain range                Δε*_(th). The strain range Δε_(ph) is derived from the                primary stress range Δσ_(ph) so as to calculate the                total strain Δε at the fillet,

Δε=Δε_(ph) +K _(t)Δε*_(th)

-   Step 6. Calculate the fatigue damage D_(f) and creep damage D_(c) of    the plate-fin heat exchanger,

$D_{f} = {\frac{N_{t}}{N_{f}\left( {\Delta \; ɛ*K_{s}} \right)}.}$

-   -   Where, N_(t) means the number of fatigue cycles.    -   N_(f)(ε) means the corresponding fatigue life on the corrected        fatigue design curve when the strain range is ε.

$D_{c} = {N_{i}*{\int_{0}^{t_{h}}{\frac{dt}{{tr}\left\lbrack {{\sigma_{e}^{*}(t)}*K_{\sigma}} \right\rbrack}.}}}$

-   -   Where, N_(i) means the number of fatigue cycles.    -   t_(h) means the strain retention time.    -   σ*_(e)(t) means the macro stress at the moment, t.    -   tr(σ) means the corresponding creep rupture life on the        corrected creep rupture design curve when the stress is σ.    -   The fatigue damage D_(f) is the ratio of the number of operating        cycles of each point of the plate-fin heat exchanger core to its        number of allowable cycles at the maximum temperature of the        aged base material. For calculating the creep damage D_(c), it        is better to get the stress relaxation curve of the structure,        namely, σ*_(e)(t) change curve, since the stress relaxation will        occur in the retention time.

-   Step 7. If D_(f)+D_(c) is less than 1, then perform step 8; if    D_(f)+D_(c) is greater than or equal to 1, then perform Step 1.    -   According to the ASME creep-fatigue damage assessment criteria,        as shown in FIG. 3, D_(f) is taken as the X-axis and D_(c) is        taken as the y-axis and D_(c)+D_(f)=1 is the envelop generated        by the cracks based on the calculated total creep damage D_(c)        and total fatigue damage D_(f). If D_(f)+D_(c) is less than 1,        namely, (D_(f), D_(c)) is below the envelope, then it means that        the plate-fin heat exchanger will not fail within its entire        design life at design temperature and pressure and satisfies the        design requirements and it is necessary to perform Step 8. If        D_(f)+D_(c) is greater than or equal to 1, it means that the        plate-fin heat exchanger does not meet the design requirements.        In this case, it is necessary to improve the structure, replace        the material, reduce the service pressure and temperature under        the allowable conditions and repeat Step 1 until (D_(f), D_(c))        is below the envelope so that the high temperature strength        design for the plate-fin heat exchanger is completed.

-   Step 8. The design for the plate-fin heat exchanger is completed.

The descriptions above are merely preferred embodiments of the presentinvention and shall not be regarded as any other form of restrictions onthe present invention and the technical contents disclosed above may bemodified or developed by any technician skilled in the art to theequivalent embodiments with equivalent change. However, any and allsimple modifications, equivalent changes and developments that are madeto the above-mentioned embodiments based on the technical essence of thepresent invention without being separated from the contents of thetechnical solutions of the present invention are still covered by theprotection scope of the technical solution of the present invention.

What is claimed is:
 1. A design method for creep-fatigue strength of a plate-fin heat exchanger, wherein, the method comprises the following steps: Step 1: preliminarily designing a plate-fin structure of the plate-fin heat exchanger based on design temperature and design pressure requirements, and defining an operating temperature, number of operating cycles and service life of the plate-fin heat exchanger; Step 2: conducting a primary stress analysis for the plate-fin structure using finite element software to identify stress concentration parts and determining an allowable stress S_(t), the stress concentration parts possessing a stress level; Step 3: determining whether the stress level of the stress concentration parts satisfies two conditions: P _(m) ≤S _(t) ; P _(L) +P _(b) ≤K _(t) *S _(t), wherein P_(m) represents primary membrane stress, P_(L) represents local membrane stress, P_(b) represents primary bending stress, S_(t) represents time-dependent allowable stress and K_(t) is a value between 1.05 and 1.16; the method comprising assessing whether the two conditions are satisfied, wherein if the two conditions are satisfied, performing Step 4, and if the two conditions are not satisfied, changing a structure and plate thickness of a core of the preliminary plate-fin heat exchanger and returning to Step 2; Step 4: conducting a creep rupture experiment and a fatigue experiment on the plate-fin structure and on an aged base material in a service environment, calculating a stress magnification factor K_(σ) and a strain magnification factor K_(s) and correcting a fatigue design curve and a creep rupture design curve for the base material according to the experimental results, the stress magnification factor K_(σ) and the strain magnification factor K_(s) being defined as: ${K_{\sigma} = \frac{\sigma_{B}}{\sigma_{B}^{*}}},{K_{s} = \frac{\Delta_{s}}{\Delta_{s}^{*}}},$ wherein, σ_(B) and σ*_(B) represent a creep rupture strength of the base material and the plate-fin structure in the same creep rupture time, respectively, Δ_(s) and Δ*_(s) represent a macro-strain range of the base material and the plate-fin structure in the same fatigue life, respectively; Step 5: acquiring equivalent mechanical parameters and equivalent thermophysical parameters of the plate-fin structure to perform a finite element analysis for thermal fatigue for the plate-fin heat exchanger, finding a time history of a micro-stress σ*_(th) of the plate-fin heat exchanger core in a height direction and calculating a total strain Δε at a fillet based on: Δε=Δε_(ph) +K _(s)Δε*_(th), wherein, Δε_(ph) represents a strain range that is derived from a stress range Δσ_(ph) obtained from the primary stress analysis, Δε*_(th) represents a ratio of the difference between a maximum value and a minimum value of a macroscopic stress σ*_(th) obtained from the thermal fatigue analysis to the elastic modulus of the plate-fin heat exchanger core in the height direction; Step 6: calculating a fatigue damage D_(f) and a creep damage D_(c) of the plate-fin heat exchanger core: ${D_{f} = \frac{N_{t}}{N_{f}\left( {{\Delta ɛ}*K_{s}} \right)}},$ wherein, N_(t) represents a number of fatigue cycles, N_(f)(ε) represents corresponding fatigue life on a corrected fatigue design curve if the strain range is ε, and ${D_{c} = {N_{i}*{\int_{0}^{t_{h}}\frac{dt}{{tr}\left\lbrack {{\sigma_{e}^{*}(t)}*K_{\sigma}} \right\rbrack}}}},$ Where, N_(i) represents the number of fatigue cycles, t_(h) represents strain retention time, σ*_(e)(t) represents a macro stress at the moment, t, tr(σ) represents corresponding creep rupture life on a corrected creep rupture design curve if the stress is σ; Step 7: performing, if D_(f)+D_(c) is less than 1, Step 8, wherein if D_(f)+D_(c) is greater than or equal to 1, then returning to Step 1; and Step 8: The design for the plate-fin heat exchanger is completed.
 2. The design method for creep-fatigue strength of the plate-fin heat exchanger according to claim 1, wherein the allowable stress S_(t) as described in Step 2 and Step 3 includes an allowable stress S_(t1) of a fin area and an allowable stress S_(t2) of a seal area.
 3. The design method for creep-fatigue strength of the plate-fin heat exchanger according to claim 1, wherein the step of acquiring the equivalent mechanical parameters and the equivalent thermophysical parameters of the plate-fin structure in Step 5 comprises substeps of: a. dividing the plate-fin heat exchanger core into several plate-fin cells of the same shape; b. considering the plate-fin cells equivalent to uniform solid plates; and c. acquiring the equivalent mechanical parameters and the equivalent thermophysical parameters of one plate-fin cell, thus obtaining the equivalent mechanical parameters and the equivalent thermophysical parameters of an entirety of the plate-fin heat exchanger core.
 4. The design method for creep-fatigue strength of the plate-fin heat exchanger according to claim 3, wherein the equivalent mechanical parameters include an anisotropic equivalent elastic modulus, an equivalent shear modulus and Poisson's ratio, and the equivalent thermophysical parameters include an equivalent thermal conductivity, an equivalent coefficient of thermal expansion, an equivalent density and an equivalent specific heat.
 5. The design method for creep-fatigue strength of the plate-fin heat exchanger according to claim 4, wherein the anisotropic equivalent elastic modulus is calculated by: establishing a coordinate system by defining a midpoint of a bottom of a front of the plate-fin heat exchanger as an origin of the coordinate system, defining a direction parallel to an axis of a flow path in a horizontal plane as an x-axis of the coordinate system, defining a direction perpendicular to the axis of the flow path as a y-axis of the coordinate system and defining a vertical direction as a z-axis of the coordinate system, and calculating ${E_{x} = {\frac{\left\{ {\left\lbrack {{l\; \tan \; \frac{a}{2}} + {dt} + {\left( {l + d} \right)\delta}} \right\rbrack - {{\delta \left( {\delta + t} \right)}{\tan \left( {45^{\circ} = \frac{a}{4}} \right)}}} \right\}}{\left\lbrack {d - {\delta \; {\tan \left( {45^{\circ} - \frac{a}{4}} \right)}} + {l\; \sin \; \frac{a}{2}}} \right\rbrack \left( {{l\; \cos \; \frac{a}{2}} + \delta + {2t}} \right)}E_{0}}},{E_{y} = {\frac{{2t} + \delta}{{l\; \cos \; \frac{a}{2}} + \delta + {2t}}E_{0}}},{E_{z} = {\frac{{\delta\left\lbrack {l + {\delta \; \tan \; \frac{a}{2}} + {\delta \; {\tan \left( {45^{\circ} - \frac{a}{4}} \right)}}} \right\rbrack}\cos \; \frac{a}{2}}{\left\lbrack {d - {\delta \; {\tan \left( {45^{\circ} - \frac{a}{4}} \right)}} + {l\; \sin \; \frac{a}{2}}} \right\rbrack \left( {{l\; \cos \; \frac{a}{2}} + \delta} \right)}E_{0}}},$ wherein E_(x), E_(y) and E_(z) represent the equivalent elastic modulus in the direction of the x-axis, y-axis and z-axis respectively, E₀ represents the elastic modulus of the base material, d represents a width of an upper side of the flow path of the plate-fin heat exchanger core, $\frac{a}{2}$ represents an angle between a side and a vertical plane of the flow path, l represents a length of the side of the flow path, t represents a thickness of a plate of the plate-fin heat exchanger core, δ represents the thickness of a fin of the plate-fin type heat exchanger core.
 6. The design method for creep-fatigue strength of the plate-fin heat exchanger according to claim 5, wherein the Poisson's ratio is calculated as follows: ${v_{xz} = {\frac{E_{z}}{E_{x}}v_{0}}},{v_{xy} = {\frac{E_{y}}{E_{x}}v_{0}}},{v_{yz} = \frac{\left\{ {{{v_{0}\left( {{2t} + \delta} \right)}\cos \; \frac{a}{2}} + {\left( {{l\; \cos \; \frac{a}{2}} + \delta} \right)\sin \; \frac{a}{2}\tan \; \frac{a}{2}}} \right\} E_{z}}{\cos \; \frac{a}{2}\left( {{2t} + \delta + {l\; \cos \; \frac{a}{2}}} \right)E_{y}}},$ wherein v_(xy) represents the ratio of x-axis strain to y-axis strain under a y-axis load, v_(xz) represents the ratio of the x-axis strain to z-axis strain under a z-axis load, v_(yz) represents the ratio of the y-axis strain to the z-axis strain under a z-axis load, v₀ represents the Poisson's ratio of the base material.
 7. The design method for creep-fatigue strength of the plate-fin heat exchanger according to claim 4, wherein the equivalent shear modulus is calculated by: establishing a coordinate system by defining a midpoint of a bottom of a front of the plate-fin heat exchanger as an origin of the coordinate system, defining a direction parallel to an axis of a flow path in a horizontal plane as an x-axis of the coordinate system, defining a direction perpendicular to the axis of the flow path as a y-axis of the coordinate system and defining a vertical direction as a z-axis of the coordinate system, and calculating ${G_{xy} = \frac{\left( {{2t} + \delta} \right)E_{0}}{2\left( {{l\; \cos \; \frac{a}{2}} + \delta + {2t}} \right)\left( {1 + v_{0}} \right)}},{G_{xz} = \frac{{\left\lbrack {d - {\delta \; {\tan \left( {45^{\circ} - \frac{a}{4}} \right)}} + {l\; \sin \; \frac{a}{2}}} \right\rbrack\left\lbrack {{\delta \; \tan \; \frac{a}{2}} + {\delta \; {\tan \left( {45^{\circ} - \frac{a}{4}} \right)}}} \right\rbrack}E_{0}}{\left( {1 + v_{0}} \right)\begin{Bmatrix} {{\left( {{2t} + \delta} \right)\left\lbrack {{\delta \; \tan \; \frac{a}{2}} + {\delta \; {\tan \left( {45^{\circ} - \frac{a}{4}} \right)}}} \right\rbrack} +} \\ {l\left\lbrack {d - {\delta \; {\tan \left( {45^{\circ} - \frac{a}{4}} \right)}} + {l\; \sin \; \frac{a}{2}}} \right\rbrack} \end{Bmatrix}}},{G_{yz} = \frac{2E_{0}{\delta^{3}\left( {{l\; \cos \; \frac{a}{2}} + \delta + {2t}} \right)}}{\begin{matrix} {{4{\delta^{3}\left( {1 + v_{0}} \right)}\left( {{2t} + \delta} \right)} +} \\ {{\left( {l - {2\; \delta}} \right)^{3}\left\lbrack {d - {\delta \; {\tan \left( {45^{\circ} - \frac{a}{4}} \right)}} + {l\; \sin \; \frac{a}{2}}} \right\rbrack}\cos \; \frac{a}{2}} \end{matrix}}},$ wherein v₀ and E₀ represent the Poisson's ratio and elastic modulus of the base material, respectively, d represents a width of an upper side of the flow path of the plate-fin heat exchanger core, $\frac{a}{2}$ represents an angle between a side and a vertical plane of the flow path, l represents the length of the side of the flow path, t represents a thickness of a plate of the plate-fin heat exchanger core, δ represents the thickness of a fin of the plate-fin type heat exchanger core.
 8. The design method for creep-fatigue strength of the plate-fin heat exchanger according to claim 4, wherein the equivalent thermal conductivity is calculated by: establishing a coordinate system by defining a midpoint of a bottom of a front of the plate-fin heat exchanger as an origin of the coordinate system, defining a direction parallel to an axis of a flow path in a horizontal plane as an x-axis of the coordinate system, defining a direction perpendicular to the axis of the flow path as a y-axis of the coordinate system and defining a vertical direction as a z-axis of the coordinate system, and calculating ${\lambda_{x} = {\frac{1}{\left\lbrack {{l\; \sin \; \frac{a}{2}} + d - {\delta \; {\sin \left( {45^{\circ} - \frac{a}{4}} \right)}}} \right\rbrack \left( {{l\; \cos \; \frac{a}{2}} + \delta + {2t}} \right)}\begin{Bmatrix} {{\lambda_{a}l\; \cos \; {\frac{a}{2}\left\lbrack {d - {2\; \delta \; \tan \; \left( {45^{\circ} - \frac{a}{4}} \right)} + {l\; \sin \; \frac{a}{2}}} \right\rbrack}} +} \\ {\lambda_{m}{\delta\left\lbrack {l - {\delta \; {\tan \left( {45^{\circ} - \frac{a}{4}} \right)}} + d} \right\rbrack}} \end{Bmatrix}}},{\frac{1}{\lambda_{y}} = {\frac{1}{{l\; \sin \; \frac{a}{2}} + d - {\delta \; {\tan \left( {45^{\circ} - \frac{a}{4}} \right)}}}\left\{ {\frac{\delta}{\lambda_{m}} + \frac{\left\lbrack {{l\; \sin \; \frac{a}{2}} + d - {\delta \; {\tan \left( {45^{\circ} - \frac{a}{4}} \right)}} - \delta} \right\rbrack \left( {{l\; \cos \; \frac{a}{2}} + \delta + {2t}} \right)}{{\lambda_{a}l\; \cos \; \frac{a}{2}} + {\lambda_{m}\left( {{2t} + \delta} \right)}}} \right\}}},{\frac{1}{\lambda_{z}} = {\frac{1}{{l\; \cos \; \frac{a}{2}} + \delta + {2t}}\left\{ {\frac{\delta + {2t}}{\lambda_{m}} + \frac{l\; \cos \; {\frac{a}{2}\left\lbrack {d - {\delta \; {\tan \left( {45^{\circ} - \frac{a}{4}} \right)}} + {l\; \sin \; \frac{a}{2}}} \right\rbrack}}{{\lambda_{a}\left\lbrack {d - {\delta \; {\tan \left( {45^{\circ} - \frac{a}{4}} \right)}} + {l\; \sin \; \frac{a}{2}} - \delta} \right\rbrack} + {\lambda_{m}\delta}}} \right\}}},$ wherein λ_(x), λ_(y) and λ_(z) represent the equivalent thermal conductivity in the direction of the x-axis, y-axis and z-axis, respectively, λ_(a) and λ_(m) represent the thermal conductivity of the base material and air, respectively, d represents a width of an upper side of a flow path of the plate-fin heat exchanger core, $\frac{a}{2}$ represents an angle between a side of the flow path and a vertical plane of the flow path, l represents the length of the side of the flow path, t represents the thickness of a plate of the plate-fin heat exchanger core, δ represents the thickness of a fin of the plate-fin type heat exchanger core.
 9. The design method for creep-fatigue strength of the plate-fin heat exchanger according to claim 4, wherein the equivalent coefficient of thermal expansion is calculated by: establishing a coordinate system by defining a midpoint of a bottom of a front of the plate-fin heat exchanger as an origin of the coordinate system, defining a direction parallel to an axis of a flow path in a horizontal plane as an x-axis of the coordinate system, defining a direction perpendicular to the axis of the flow path as a y-axis of the coordinate system and defining a vertical direction as a z-axis of the coordinate system, and calculating ${a_{z} = {\frac{a_{0}}{{l\; \cos \; \frac{a}{2}} + {2t} + \delta}\left\{ {{2t} + \frac{\begin{matrix} {{\left( {{l\; \sin \; \frac{a}{2}} + {\delta \; \tan \; \frac{a}{2}}} \right)\delta^{2}} +} \\ {{\left\lbrack {d - {2\; \delta \; {\tan \left( {45^{\circ} - \frac{a}{4}} \right)}}} \right\rbrack\left\lbrack {d - {\delta \; {\tan \left( {45^{\circ} - \frac{a}{4}} \right)}} + {l\; \sin \; \frac{a}{2}}} \right\rbrack}\delta} \end{matrix}}{\begin{matrix} {{\left\lbrack {d - {2\; \delta \; {\tan \left( {45^{\circ} - \frac{a}{4}} \right)}}} \right\rbrack\left\lbrack {d - {\delta \; {\tan \left( {45^{\circ} - \frac{a}{4}} \right)}} + {l\; \sin \; \frac{a}{2}}} \right\rbrack} +} \\ {\delta^{2}\tan \; \frac{a}{2}} \end{matrix}}} \right\}}},\mspace{79mu} {a_{y} = a_{0}},\mspace{79mu} {a_{x} = a_{0}},$ wherein a₀ represents the equivalent coefficient of thermal expansion of the base material, d represents a width of an upper side of a flow path of the plate-fin heat exchanger core, $\frac{a}{2}$ represents an angle between a side of the flow path and a vertical plane, l represents a length of the side of the flow path, t represents a thickness of a plate of the plate-fin heat exchanger core, δ represents the thickness of a fin of the plate-fin type heat exchanger core.
 10. The design method for creep-fatigue strength of the plate-fin heat exchanger according to claim 4, wherein, the equivalent density and the equivalent specific heat are calculated as follows: ${\beta_{a} = \frac{{l^{2}\sin \; \frac{a}{2}\cos \; \frac{a}{2}} + {l\; \cos \; {\frac{a}{2}\left\lbrack {d - {2\delta \; {\tan \left( {45^{\circ} - \frac{a}{4}} \right)}}} \right\rbrack}}}{\left( {{l\; \cos \; \frac{a}{2}} + \delta + {2t}} \right)\left\lbrack {d + {l\; \sin \; \frac{a}{2}} - {\delta \; {\tan \left( {45^{\circ} - \frac{a}{4}} \right)}}} \right\rbrack}},{\beta_{m} = {1 - \beta_{a}}},{\overset{\_}{\rho} = {{\beta_{m}\rho_{m}} + {\beta_{a}\rho_{a}}}},{c_{p} = \frac{{\left( {1 - \beta_{a}} \right)\rho_{m}c_{p\; 1}} + {\beta_{a}\rho_{a}c_{p\; 2}}}{{\left( {1 - \beta_{a}} \right)\rho_{m}} + {\beta_{a}\rho_{a}}}},$ wherein β_(m) and β_(a) represent a base material volume fraction and an air volume fraction, respectively, ρ_(m) and ρ_(a) represent a base material density and an air density, respectively, c_(p1) and c_(p2) represent a base material specific heat and an air specific heat, respectively, c_(p) and ρ represent the equivalent specific heat and the equivalent density, respectively, d represents a width of an upper side of a flow path of the plate-fin heat exchanger core, $\frac{a}{2}$ represents an angle between a side and a vertical plane of the flow path, l represents a length of the side of the flow path, t represents a thickness of a plate of the plate-fin heat exchanger core, δ represents a thickness of a fin of the plate-fin type heat exchanger core. 